Various known imaging techniques utilizing pixel array detectors, such as CCD, photovoltaic cell, as well as those based on photoemission, typically require the use of color filters in order to obtain a colored picture (i.e., Red, Green, Blue pixels). A full-spectrum image (from IR to UV) can be obtained by combining IR and UV range detectors visible light detectors.
IR detection (in the range of about 1-10 μm), and specifically far-IR detection (8-12 μm), is typically realized in two main approaches. The first approach utilizes Photonic Detectors, namely detectors that are based on the photoelectric effect. The conventional photonic detectors, although being considered today as the best IR detectors, suffer from the unavoidable need for cooling. The second approach utilizes Thermal Detectors, which are detectors that change their temperature in response to absorbed energy. These detectors are insensitive to the wavelength of photons. Two photons of 0.5 eV will have the same effect as one photon of 1 eV. Therefore, filters are used to narrow the bandwidth. Since the detectors are sensitive to a change in their temperature, the requirement for operating with these detectors includes stabilizing the temperature of the detectors and the surroundings to a fixed temperature.
U.S. Pat. No. 3,814,993 discloses a tunable infrared photocathode. The photocathode is structured as a three layer double heterojunction device with a low work function cesium oxide coating on the electron emitting surface. An internal field assistance bias aids the flow of electrons from a narrow bandgap region, where they are photo-generated, to the wider bandgap negative electron affinity surface region for vacuum emission. Here, photons are absorbed in narrow-gap III-V materials, and then electrons are transported via an electric field to a negative electron affinity photocathode. A problem with this technology is associated with the fact that patterns (arrays of electrodes) made from these materials are difficult to manufacture. Moreover, the device suffers from a low sensitivity at room temperature, “real time” tunability is hard to achieve, and “offline” tunability is achieved only through changing the layers composition during the manufacturing stage.
U.S. Pat. No. 4,644,221 discloses a variable sensitivity transmission mode negative electron affinity photocathode, and method of its manufacture. Here, the sensitivity of the photocathode to white or monochromatic light can be varied by varying the back surface recombination velocity of the photoemitting material with an electric field. The basic structure of the photocathode is comprised of a Group III-V element photoemitter on a larger bandgap Group III-V element window substrate. According to this technique, because of surface recombination, some of photoelectrons are prevented from being emitted, and thus an increase of surface recombination results in a decrease of sensitivity. With this technique wavelength tunability is hard to achieve.
U.S. Pat. No. 5,384,469 describes voltage-tunable, multicolor infrared detectors. The detector comprises a superlattice structure having a plurality of quantum well units each separated by a first potential barrier and each having at least two doped quantum wells separated by a second potential barrier. Each of the wells has a lower energy level and a higher energy level. The first potential barriers substantially impede electrons at the lower levels from tunneling therethrough. The second potential barriers permit electrons at the lower levels to tunnel therethrough and prevent energy-level coupling between adjacent ones of the doped quantum wells. A biasing circuit is connected across the semiconductor superlattice structure. A photocurrent sensor is provided for measuring the amount of radiation absorbed by the semiconductor superlattice structure. The superlattice structure is made a part of a hot-electron transistor for providing amplification. Such a Quantum Well Infrared Photodetector (QWIP) typically has poor sensitivity because, as a result of quantum selection rules, only light propagating along the well plane and not perpendicular thereto is absorbed. The spectral sensitivity of the detector is limited to a fairly narrow spectral range.